https://leetcode-cn.com/problems/reverse-integer/

class Solution {
public:
    int reverse(long long int x) {
       bool flag=false;
       if(x<0) flag=true,x=-x;
       long long int ans=0;
       while(x)
       {
           ans=ans*10+x%10;
           x/=10;
       }
       if(flag) ans=ans*-1;
       if((ans>pow(2,31)-1)||ans<(-1*pow(2,31))) return 0;//过界了
       return ans;
    }
};